1. Field of the art
This invention relates generally to thermal cracking of heavy hydrocarbons and more particularly to a process for thermally cracking heavy hydrocarbons (hereinafter referred to as "heavy petroleum oils") by causing the heavy petroleum oils to contact particles fluidized by principally steam thereby to obtain principally light hydrocarbons (hereinafter referred to as "light petroleum oils") which are in liquid state at room temperature.
2. Prior art
Among the techniques heretofore known of thermally cracking heavy petroleum oils by means of fluidized beds, there are the fluid coking process, and cracking process, and others. In these processes coke formed by thermal cracking, river sand, and other particles are used as fluidized particles. In all cases, there are almost no fine holes or pores in these particles, that is, they are so-called non-porous material, and relatively coarse particles of weight-mean diameters of 0.3 mm or more are being used.
In the case where thermal cracking of a heavy petroleum oil is to be carried by means of a fluidized bed comprising non-porous powder materials of this nature, the surfaces of the particles become covered with polymers having tackiness which are produced as the thermal cracking proceeds. As a consequence, when the amount of carbon or coke deposited on the particles becomes large, the particles agglomerate, and the fluidity becomes inadequate. This phenomenon is commonly called "bogging", and must be absolutely avoided in thermal cracking of heavy petroleum oils by means of a fluidized bed.
As the particle size of the fluidized particles is made coarse, the fluidization will become vigorous if the velocity of the fluidizing fluid is increased, whereby the tendency of bogging to occur is reduced. This is the reason why, in the known processes, only fluidized beds of relatively coarse particles (hereinafter referred to as "fluidized coarse-particle bed(s)") have been used.
However, it is well known that in a fluidized coarse-particle bed of this character, the fluidized state is extremely ununiform because of causes such as the generation of large bubbles and slugging, and not only is the progress of the reaction obstructed, but trouble such as loss of the fluidized particles due to attrition and damage or breakage of th apparatus due to adrasive wear easily occur.
On the other hand, it is also well known that a fluidized bed comprising relatively fine particles (hereinafter referred to as "fluidized fine-particle bed(s)"), in comparison with the above described fluidized coarse-particle bed, exhibits a very uniform fluidized state, and the reaction progresses smoothly, undesirable results such as attrition of the fluidized particles and abrasive wear of the apparatus being greatly reduced. In a fluidized fine-particle bed, however, bogging readily occurs since the fluidization is weak, whereby if the bed is used as it is, thermal cracking of a heavy petroleum oil cannot be accomplished.
A possible measure for solving the above described problems will now be considered. Relative to the problems accompanying fluidized fine-particle beds of this character, we have previously proposed a process, as disclosed in the specification of Japanese Pat. Appln. No. 84543/1979.
The process according to this preceding invention has succeeded in carrying out with good efficiency thermal cracking of heavy petroleum oils by means of a fluidized fine-particle bed by using fine particles of specific grain size and shape and, moreover, adding a process step of oxidative regeneration of the used fine-particles. By the process according to this preceding invention, since the quantity of recirculated fine particles can be remarkably increased, the quantity of coke deposited on the particles of the thermal cracking step is greatly reduced.
A feature of this preceding invention is that cracking of heavy petroleum oils can be carried out even at a medium temperature, that is, in the vicinity of 500.degree. C. However, if thermal cracking of heavy petroleum oils could be accomplished at even lower temperatures, and, furthermore, if the quantity of recirculated fine-powder particles could be reduced, it would be even more advantageous.